Technical Abstract:
Swine manure lagoon effluent is a valuable resource in the Mid South US and is applied as fertilizer for hay. Levels of nutrients have been widely studied in lagoon effluent and in fertilized soil and levels of fecal bacteria in effluent, including potential human pathogens, are also known. Less is known about these bacteria in irrigated ‘spray field’ soils.
A team of Agricultural Research Service scientists in USDA’s Crop Science Research Laboratory at Mississippi State, Mississippi, investigated the nutrients and bacteria in soils of spray fields that had been in use for more than 15 years. The scientists collected soil from inside irrigated fields and from the same soil types in adjoining areas outside the spray fields. They compared the levels of nutrients and bacteria inside and outside spray fields across twenty soil types on five farms. Results from the study were published in the September-October 2010 issue of the Journal of Environmental Quality. The research was also presented in Pittsburgh, PA, at the 73rd Annual Meeting of the Soil Science Society of America in November 2009. Soils were sampled in February and March 2009, between irrigation seasons. Nutrient tests showed higher pH and higher levels of phosphorus, potassium, magnesium, sodium, copper, and zinc inside than outside spray fields. Levels of nitrogen, carbon, calcium, and manganese did not differ. Results were consistent with what was expected for spray fields after long-term use. Differences between the same soil types inside and outside confirmed the hypothesis that outside soils had not received effluent and would provide good comparisons of bacteria. Several bacterial groups were higher inside. These included heterotrophic plate counts (a measure of total bacteria which use organic carbon), thermotolerant coliforms (a measure of fecal bacteria), Staphylococcus (commensal bacteria common on animals and potential human pathogens) and Clostridium perfringens (common gut inhabitants and potential pathogens). Levels of Escherichia coli (a fecal indicator and common gut bacteria with some pathogenic strains) and Enterococcus (also a fecal indicator and potential pathogen) were not different. Listeria, an enteric pathogen known to occur in effluent was cultured from 23% of samples inside and 28% outside. Enteric pathogens, Campylobacter and Salmonella, also known in effluent, could not be cultured from any soil samples. DNA tests, capable of detecting even dead bacteria, detected these three enteric pathogens at low levels that were not different inside and outside. The investigators also analyzed public health data from three public health districts with similar land areas, populations, and agricultural bases, but with different numbers (zero to 44) of swine CAFOs (confined animal feeding operations). Their analysis of annual reports of illnesses caused by Campylobacter and Salmonella from 1993 through 2008 showed no relationship between reported cases of these human illnesses and swine CAFO numbers. Team leader, Mike McLaughlin, stated “Finding low levels of pathogens outside spray fields is not surprising, because these bacteria are known to infect a wide range of wild and domestic birds and animals”. Team microbiologist, John Brooks, added “Finding similar low levels inside and outside the spray fields suggests that the low levels of pathogens in effluent are further diluted in spray fields and either do not survive in soil or survive at low levels below cultural detection limits”. This first report on spray field bacteria in the region suggests that manure nutrient management plans have been effective for nutrients and for bacterial pathogens. Future research will focus on enhanced resolution of pathogen levels in effluents and soils, on pathogen survival and transport in soil and on plants, and on practical solutions to further reduce or eliminate risks from these pathogens.